How to fix obvious connection line between meshes?
It look like you use 2.79 on one screenshot.
If you want to output it then somewhere from Blender - bake normal maps from models where all parts attached and vertices merged to each separate part. This normal map will be baked with taking in to account normal differences so it will compensate it and make detached parts look similar to full head.
If use just in Blender - use Normal Edit modifier on detached parts where pick full head model with merged vertices as reference object. Like on GIF - 2 separate sphere parts, &quot; normal Object &quot; is full sphere.

I mean use modifier for &quot; non destructive &quot; things. You can also just apply this modifier and not bake any normal maps.